Helicobacter pylon is classified by IARC/WHO as a definite human gastric carcinogen, despite "inadequate experimental evidence". To obtain direct evidence concerning this relationship, we investigated the histopathological findings of gastric mucosa using a model of H.pylori infection in Mongolian gerbils. The animals were challenged p. o. with H.pylori ATCC-43504 and sacrificed at 1, 2, 3, 6, 12, and 18 months after inoculation for histological examination. The spiral bacteria were observed in the mucus and gastric pits of all infected animals.A severe infiltration of the lamina propria by polymorphonuclear and mononuclear cells was seen 1 month after H.pylori inoculation. The submucosa was infiltrated by mainly mononullear cells with formation of lymphoid follicles 2 and 3 months after inoculation. Gastric ulcer at the pyloric area in proximity to the fluidus, atrophic gastritis and focal intestinal metaplasia in the lesser curvature of the antrum appeared at 6 months after inoculation.Intestinal metaplasia became severe, with dysplasia, after that. At 18 mnoths after H.pylori inoculation, two of five infected animals showed three well-differentiated gastric adenocarcinomas. The uninfected control animals showed no abnormal findings throughout the entire observation period.This study shows gastric carcinogenesis in a H.pylori-infected Mongolian gerbil model and confirms the IARC/WHO report that H.pylori is a definite human gastric carcinogen.